The varieties of language experience

Yes, I am borrowing the title of this post from William James because there are many experiences in both religion and language. Probably as many as there are people on earth. In the case of language, the final goal is communication, but there are many ways to achieve this end. In fact, there are 6500 different languages in the world. 6500 methods for brains to get in touch with other brains. And each language has its own varieties, and propably each variety has yet another variety. How many varieties of Spanish are there? Which one should I try to learn? Goodness gracious! The only answer I would venture to give is that the best variety is the one easiest to understand. After all, the idea behind language is to be understood.